(1) Field of the Invention
The present invention relates to a method of producing an oxygen concentration cell which makes use of oxygen ion conductive solid electrolyte.
(2) Description of the Prior Art
There has been well known an oxygen concentration cell consisting of a partition wall made of oxygen ion conductive solid electrolyte, such as stabilized zirconia or the like, and electrically conductive electrodes applied to both of the opposite surfaces of the wall. It is widely known that this oxygen concentration cell is used for the measurement of oxygen partial pressure in the waste gas of boiler or in the exhaust gas of motor car, and is used as a fuel cell for the direct generation of electricity. When the oxygen concentration cell is used for the above described purposes, it is very important that the electrodes of the cell are formed of durable electrodes having a high activity and a low internal resistance.
In general, porous platinum group metals are used as a material for the electrodes of the oxygen concentration cell of this kind.
As the method of applying the porous electrodes formed of platinum group metal to the solid electrolyte, chemical plating method, physical plating method, baking method and the like are known. However, the electrode formed by the chemical plating method has the serious drawbacks that the electrode is poor in durability due to its poor adhesion to the solid electrolyte, and that since the electrode is formed into a filmly layer, the electrode is low in the gas permeability and activity. The electrode formed by the physical plating methods, such as vapor deposition, sputtering, ion plating and the like, has the drawbacks that an expensive physical plating apparatus, which has a complicated structure and is poor in workability, must be used, that the material for the electrode is wastefully adhered to the inner wall or the like of the apparatus, and that since the electrode is formed into a filmy layer, the electrode is poor in gas permeability and in activity and is not sufficient in durability.
Further, the electrode formed by the baking method, wherein a paste formed of platinum powder is appliedto the solid electrolyte and baked thereto, has the drawbacks that the electrode has a high internal resistance, and that when a paste containing glass component is used, the activity of the electrode is low, while when a paste containing no glass component is used, the electrode is poor in adhesion to the solid electrolyte and in durability.
Recently, a method of forming electrodes has been proposed, wherein the surfaces of a solid electrolyte are roughed to form coarse surfaces, a solution of platinate is impregnated into pores of the coarse surfaces, and the platinate is reduced to form the electrodes. However, in this method, since the diffusion velocity of gas into the platinum phase deposited in the pores on the solid electrolyte surface is low, the response speed of variation of electromotive force against the variation of oxygen partial pressure in a gas to be measured is low where the resulting cell is used for the measurement of oxygen partial pressure. Further, during the platinate applied to the solid electrolyte in the form of a solution is reduced, the platinate solution tends to flow down, and a platinum layer having a uniform thickness or a thick platinum layer can not be obtained.